Braid construction to match coefficients of thermal expansion for composite transmission housings

ABSTRACT

A transmission assembly includes a composite transmission component with a tailored coefficient of thermal expansion (CTE). The composite transmission component is fabricated from graphite axial fibers and S-glass bias fibers that are oriented at a bias angle to the graphite axial fibers. The composite transmission component is located adjacent to a second composite transmission component and a metal transmission component. A first CTE of the composite transmission component is tailored to a CTE of the second composite component. A second CTE of the composite transmission component is tailored to a metal CTE of the metal transmission component. The first and second CTE are tailored by utilizing graphite axial fibers and S-glass bias fibers, or two other different types of fibers, and by controlling the bias angle. The tailored first and second CTE minimize thermal strain and maintain a tight fit between the components.

BACKGROUND OF THE INVENTION

This invention relates to transmission housings and, more particularly, to a composite transmission housing component with a tailored coefficient of thermal expansion (CTE).

Conventional power transmissions and transmission components may potentially use light-weight composite materials to reduce the weight of the power transmission, however, some components such as gears, bearing liners, and bearing races are still made from metal materials.

The use of composite components and metallic components in intimate contact with each other may present problems over the operating temperature range of the power transmission. Typically, the composite component has a different CTE than the metal component. The difference in CTE causes the composite component to expand and contract over a temperature range differently than the metal component expands and contracts. When the composite component and metal component are bonded together or otherwise meet at a composite-metal interface, the difference in CTE may cause thermal strain between the composite component and the metal component, which in turn may lead to a failure at the composite-metal interface. Common interface failures include physical separation between the composite component and metal component, aggravated fatigue and creep, formation of leak paths, and loosening of press fits, all of which may affect the proper functioning of the power transmission. The problem is further compounded with an additional composite-composite interface when another composite component with yet another CTE is located next to the first composite component.

Some existing conventional composite materials have been designed with a CTE that is closer to the CTE of most metals. Reducing the difference between the CTE of the composite and the CTE of the metal may alleviate the thermal strain and may reduce risk of failure at a composite-metal interface. These conventional composites typically include stacking positive and negative CTE composite layers, using specialized and expensive fibers, or orienting the reinforcing fibers to tailor the CTE in a single direction.

Although these conventional composites may alleviate thermal strain problems for composite-metal interfaces, a conventional composite may actually aggravate thermal strain problems in a power transmission application where the composite component also interfaces with another different type of composite component by increasing the difference between the CTE's of the two composite components. Despite these conventional composites, a demand remains for a composite that is thermally compatible with both metals and other composites.

Accordingly, a composite component having a CTE that more effectively matches the CTE of an interfacing metal component and the CTE of another interfacing composite component is needed.

SUMMARY OF THE INVENTION

The transmission assembly according to the present invention includes a composite transmission component with a tailored coefficient of thermal expansion (CTE). The composite transmission component is located adjacent to another composite component and a metal component and is thermally compatible with both the other composite component and the metal component.

In each of the transmission assembly examples considered, the composite transmission component is fabricated from braid employing graphite axial fibers and S-glass bias fibers that are oriented at a bias angle to the graphite axial fibers. This configuration produces a structure with two directionally dependant coefficients of thermal expansion. The composite transmission component is located adjacent to a second composite transmission component and a metal transmission component. A first CTE of the composite transmission component is tailored to a CTE of the second composite component and a second CTE of the composite transmission component is tailored to a metal CTE of the metal transmission component. The first and second CTE are tailored by utilizing graphite axial fibers and S-glass bias fibers, or two other different types of fibers, and by controlling the bias angle.

In one composite transmission component example, the composite transmission component is positioned radially inward from the metal transmission component. The first CTE is tailored to be generally equal to the CTE of the second composite component in an axial direction and the second CTE is tailored to be slightly greater than or equal to the CTE of the metal transmission component in a hoop direction to minimize thermal strain between all the components and to maintain a tight fit between the composite transmission component and the metal transmission component.

In another composite transmission component example, the composite transmission component is positioned radially outward from the metal transmission component. The first CTE is tailored to be generally equal to the CTE of the second composite component in an axial direction and the second CTE is tailored to be slightly greater than or equal to the CTE of the metal transmission component in a hoop direction to minimize thermal strain between all the components and to maintain a tight fit between the composite transmission component and the metal transmission component.

The transmission according to the present invention provides a composite component that is thermally compatible with another composite component and a metal component.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows.

FIG. 1 illustrates a schematic perspective view of an example vehicle with a transmission housing;

FIG. 2 illustrates a perspective cross-sectional view of the transmission of FIG. 1;

FIG. 3 illustrates a perspective cross-sectional view of a tail take-off drive of the transmission of FIG. 1; and

FIG. 4 illustrates a schematic view of a triaxially braided fiber reinforced composite.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 schematically illustrates a rotary-wing aircraft 10 having a main rotor assembly 12. The aircraft 10 includes an airframe 14 having an extending tail 16 which mounts an anti-torque rotor 18. The main rotor assembly 12 is driven through a transmission (illustrated schematically at 20) by one or more engines 22. Although a particular helicopter configuration is illustrated in the disclosed embodiment, other machines such as turbo-props, tilt-rotor and tilt-wing aircraft will also benefit from the present invention.

Referring to FIG. 2, the transmission 20 includes a transmission housing 30. The transmission housing 30 includes a composite outer structure 32 which is preferably fabricated from a graphite fiber reinforced composite. The composite outer structure 32 structurally supports an adjacent first composite support 34 and tail rotor take off drive 36.

The first composite support 34 is annular in shape and defines an axis 38. A bull gear 40 with a steel bearing races 41 is mounted on the first composite support 34. The bull gear 40 is located radially outward from the first composite support 34 such that the bull gear 40 and first composite support 34 meet at an interface 42. The interface 42 extends in a circumferential hoop direction 44 relative to the axis 38. The composite outer structure 32 and first composite support 34 meet at an interface 46 which extends primarily in an axial direction that is generally parallel with the axis 38.

Referring to FIG. 3, the tail rotor take off drive 36 includes a drive gear 56 defining an axis 58. Steel bearing races 60 and titanium or steel metal bearing liners 62 are mounted radially inward relative to a second composite support 64 such that the bearing races 60, bearing liners 62, and second composite support 64 meet at an interface 66. The interface 66 extends in a circumferential hoop direction 68 relative to the axis 58. The composite outer structure 32 and second composite support 64 meet at an interface 70 which extends primarily in an axial direction that is generally parallel the axis 58.

FIG. 4 illustrates a schematic view of a composite. The first composite support 34 and second composite support 64 are fabricated from an interwoven or braided composite configuration 80. Preferably the composite configuration 80 is an interwoven triaxially braided composite that includes axial fibers 82 and bias fibers 84 in a resin matrix 86. The bias fibers 84 are oriented at a bias angle 88 relative to a longitudinal axis 90 defined by the axial fibers 84. The bias angle 88 is between 0° and +/−90° relative to the axial fibers 82, and preferably is between +/−55° and +/−70°.

The axial fibers 82 are different from the bias fibers 84. In one example, the axial fibers 82 are graphite PAN fibers and the bias fibers 84 are S-glass fibers. The volumetric ratio of S-glass fibers to graphite PAN fibers is 2:1. Other types of fibers, such as Kevlar™, could also be used. The difference in the types of fibers used for the axial fibers 82 and bias fibers 84 coupled with the bias angle 88 allows the coefficient of thermal expansion (CTE) of the composite configuration 80, and thus the first composite support 34 and second composite support 64 that are fabricated from the composite configuration 80, to be tailored in certain desired directions.

In one example, the first composite support 34 has a first CTE in the axial direction relative to the axis 38 and a different second CTE in the hoop direction 44. The first CTE is controlled by aligning the longitudinal axis 90 of the graphite PAN axial fibers 82 with the axis 38. The second CTE is controlled by the bias angle 88 of the bias fibers 84. A bias angle 88 closer to +/−55° in the preferred +/−55° to +/−70° range results in a larger second CTE and a bias angle 88 closer to +/−70° in the preferred range results in a smaller second CTE. The second CTE can therefore be tailored to a desired CTE during fabrication of the first composite support 34.

The first CTE of the first composite support 34 is generally equal to an outer composite CTE of the composite outer structure 32 to minimize thermal strain produced at the interface 46. Preferably the first CTE is within about +/−10% of the outer composite CTE.

The second CTE of the first composite support 34 is generally equal to or greater than a metallic CTE of the bearing races 41 to maintain a tight fit between the first composite support 34 and bearing races 41. That is, the first composite support 34 will expand at a rate equal to or greater than the metal bearing races 41 when the transmission housing 30 is heated and exert a force on the bearing races 41 at the interface 42 to prevent separation at the interface 42. Preferably the second CTE is greater than or about equal to the metallic CTE. That is, within about +10% of the metallic CTE is considered to be about equal. This may minimize the thermal strain at the interface 42 and provide a continuously tight fit between the first composite support 34 and bearing races 41 over an operating temperature range from about −40° F. to about 300° F. Alternatively, the second CTE of the first composite support 34 may be less than or about equal to the metallic CTE, however, this is thought to produce less desirable thermal strain and tightness of fit conditions at the interface 42.

In another example, the second composite support 64 has a first CTE in the axial direction relative to the axis 58 and a different second CTE in the hoop direction 68. The first CTE is generally equal to the outer composite CTE of the composite outer structure 32 to minimize thermal strain produced at the interface 70. Preferably the first CTE is within about +/−10% of the outer composite CTE.

The second CTE of the second composite support 64 is generally equal to or greater than a metallic CTE of the bearing races 60 and bearing liners 62 to maintain a tight fit between the second composite support 64, bearing races 60 and bearing liners 62. That is, the bearing races 60 and bearing liners 62 will expand at a rate equal to or less than the second composite support 64 when the transmission housing 30 is heated. The second composite support 64 will exert a force on the bearing races 60 and bearing liners 62 at the interface 66 to prevent separation at the interface 66. Preferably the second CTE is greater than or about equal to the metallic CTE. That is, within about +10% of the metallic CTE is considered about equal. This may minimize the thermal strain at the interface 66 and provide a continuously tight fit between the second composite support 64, bearing races 60 and bearing liners 62 over an operating temperature range from about −40° F. to about 300° F. 6.

The transmission housing 30 may be fabricated using several different methods of resin transfer molding or other molding process to incorporate the first composite structure 34 and second composite structure 64. In one example, the first composite structure 34 and second composite structure 64 are each fabricated in a braiding processes. The braiding process itself is known. The axial fibers 82 and bias fibers 84 are braided over a reusable mandrel, infused with the resin matrix 86, and cured in the resin transfer molding process. The cured first composite structure 34 and cured second composite structure 64 are then positioned as molded-in inserts in a resin transfer molding process or other molding process of the composite outer structure 32.

In another example, the first composite structure 34 and second composite structure 64 are each fabricated in a braiding process. The braiding process itself is known. The axial fibers 82 and bias fibers 84 are braided over a reusable mandrel, positioned in a resin transfer mold tooling of the composite outer structure 32, infused with the resin matrix 86, and cured. Thus, the first composite structure 34, second composite structure 64 and composite outer structure 32 are infused and cured simultaneously as a single integral piece.

Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention. 

1. A composite article comprising: a plurality of first fibers having a first orientation and controlling a first coefficient of thermal expansion (CTE) in a first direction; a plurality of second fibers adjacent to said plurality of first fibers, said plurality of second fibers having a second orientation and controlling a second CTE in a second direction which is different than said first direction, said plurality of first fibers being different than said plurality of second fibers.
 2. The article as recited in claim 1, wherein said first direction is in a first plane and said second direction is in a second plane that is different from said first plane.
 3. The article as recited in claim 1, wherein said first CTE is different than said second CTE.
 4. The article as recited in claim 1, wherein said plurality of second fibers are interwoven with said plurality of first fibers.
 5. The article as recited in claim 1, wherein said plurality of first fibers comprise glass fibers.
 6. The article as recited in claim 1, wherein said plurality of first fibers comprise Kevlar™ fibers.
 7. The article as recited in claim 1, wherein said plurality of second fibers comprise graphite fibers.
 8. The article as recited in claim 1, wherein said plurality of first fibers have a volumetric ratio of about 2:1 to said plurality of second fibers.
 9. The article as recited in claim 1, wherein said first orientation comprises an angular bias between essentially 0° and +/−90° relative to an axis and said second orientation is generally parallel to said axis.
 10. The article as recited in claim 9, wherein said angular bias is between +/−55° and +/−70°.
 11. A transmission assembly comprising: a first transmission component having a first CTE in an axial direction; a second transmission component adjacent to said first transmission component, said second transmission component having a second CTE in said axial direction and a third CTE in a non-axial direction, said third CTE being different than said second CTE; and a third transmission component adjacent to said second transmission component, said third transmission component having a fourth CTE in said non-axial direction.
 12. The assembly as recited in claim 11, wherein said second CTE is generally equal to said first CTE.
 13. The assembly as recited in claim 11, wherein said third CTE is greater than said fourth CTE.
 14. The assembly as recited in claim 11, wherein said third CTE is less than said fourth CTE.
 15. The assembly as recited in claim 11, wherein said third transmission component is located radially inward from said second transmission component.
 16. The assembly as recited in claim 11, wherein said third transmission component is located radially outward from said second transmission component.
 17. The assembly as recited in claim 11, wherein said non-axial direction is a hoop direction.
 18. The assembly as recited in claim 11, wherein said first transmission component is a graphite composite case, said second transmission component is a composite support, and said third transmission component is a metal bearing component.
 19. A method of tailoring a coefficient of thermal expansion (CTE) of a composite comprising: (a) orienting a plurality of first and second fibers in a first and second orientation, respectively, to form a composite; (b) controlling a first CTE of the composite with the plurality of first fibers; and (c) controlling a second CTE of the composite with the plurality of second fibers.
 20. The method as recited in claim 19, wherein the step (a) includes providing glass fibers and graphite fibers as the plurality of first and second fibers, respectively.
 21. The method as recited in claim 19, wherein the step (a) includes orienting the plurality of first fibers with an angular bias to an axis and orienting the plurality second fibers parallel to the axis.
 22. The method as recited in claim 19, including the steps of controlling the first CTE to be equal to or greater than a CTE of a metal component adjacent to the composite and controlling the second CTE to be generally equal to a composite component adjacent to the composite.
 23. The method as recited in claim 19, including the steps of controlling the first CTE to be equal to or less than a CTE of a metal component adjacent to the composite and controlling the second CTE to be generally equal to a composite component adjacent to the composite. 